We propose to develop the next generation of x-ray detectors for analytical electron microscopy that will offer larger solid angle (thus higher sensitivity), higher count rate capability, and higher detection efficiency, to match the next generation of analytical electron microscopes which will offer the opportunity for unprecedented analytical measurements in biological applications. Smaller areas (<1 nm spatial resolution), for structural and chemical analysis, and higher sensitivities than ever before will be possible. One of the standard analytical tools on almost all electron microscopes (EM) is an energy dispersive spectroscopy (EDS) detector used for chemical analysis. However, there are many limitations with the current generation of EDS detectors for EM and TEM. The proposed new EDS detectors will be based on the silicon multi-cathode detectors (SMCD) that Photon Imaging has been developing for a number of different x-ray fluorescence (XRF) and x-ray diffraction (XRD) applications. Unlike with traditional Si(Li) or HPGe EDS detectors, these new detectors do not require liquid nitrogen, which makes them extremely compact and very flexible in their mechanical design, which can open up new possibilities in measurements capabilities. In addition, they are capable of operating at much higher count rates than traditional EDS detectors. Recently, we have begun to develop very thick SMC detectors (up to 1.5 mm thick). This extra thickness (beyond the standard 0.35 mm) offers an extended detection efficiency for hard x-rays, up to 40 keV. In Phase I, we will evaluate the x-ray performance of the new, 10-20 mm2, high efficiency SMCD detectors for TEM applications. The spectrometer will be designed to fit tightly into the EM probe area, to maximize the solid angle for signal collection, while not interfering with the pole piece nor other analytical equipment. The detector and spectrometer will be thoroughly evaluated at Photon, using radioisotope sources and an x-ray generator, and then will be evaluated on the EM, in conjunction with Dr. Nestor Zaluzec, who will consult on this project. In Phase II, we will develop larger area 50 mm2 detectors, the full prototype commercial spectrometer, with full data acquisition software, for insertion into commercially available TEMs and other types of EMs.